In a typical wireless cellular network, such as 3rd generation partnership project long term evolution (3GPP LTE), when deployed as a single frequency network (SFN), the transmissions from various cells and user equipment (UE) should be properly coordinated to maximize the overall system capacity and coverage. A cell may be defined as a coverage area provided by one or more antennas of a base station. Each base station may provide coverage for a plurality of cells via individual cell-site equipment with a common interface to an evolved packet core (EPC). For example, a base station may have antennas affixed to a tower, each of one or more antennas providing coverage for a different geographic area or cells accessible from the tower. A cell may be a serving cell or a neighboring cell. The serving cell is the cell for which the UE is radio resource controlled (RRC) or currently camped, whereas neighboring cells are nearby cells. Note that some cells may overlap and the signals transmitted in one cell may interfere with signals transmitted in another cell. The interfering cells may belong to the same base station or different base stations. Thus, in cases of cell overlap, UEs may receive signals from a plurality of cells and these signals may interfere, especially if the signals of the different cells are transmitted on the same frequency, which is the case for SFNs.
Inter-cell coordination procedures may be employed to improve signal quality experienced by a UE. Examples of inter-cell coordination procedures include inter-cell interference coordination (ICIC), down link (DL) and up link (UL) coordinated multi-point (CoMP) transmission and communication through dual-connectivity. These procedures have been studied extensively as part of the third generation partnership project (3GPP) long term evolution (LTE) advanced standardization activity. Such procedures shall be referred to herein collectively as inter-cell coordination procedures. It is known that system capacity on the down link (DL) can be improved by these procedures by constraining the inter-cell interference experienced by the UE or by increasing the useful signal content of the signal received by the UE.
For ICIC, the radio resources are coordinated between the neighboring cells such that the inter-cell interference experienced by the mobile station or UE is minimized. The coordination may include reducing the transmit power or muting specified radio resources on an interfering cell. Similarly on the up link (UL), the transmissions from the UEs can be scheduled on specific resources such that the interference at a serving cell can be reduced.
For CoMP, the transmissions from the cells can be coordinated such that the combined signal from these cells provides better signal quality at the UE. Similarly, a UE transmission can be scheduled to be received at multiple network nodes such that the received signals can be combined coherently to improve the received signal quality.
For dual connectivity, where a UE is allowed active association with multiple cells at the same time, the serving cell can dynamically select one of the cells as the serving cell to efficiently offload the data and control signaling. In this case the mobility between the coordinating cells will be very efficient.
In each of these cases, the UEs and cells which may benefit from inter-cell coordination should be identified. UEs which are at the cell edge can typically obtain gains from inter-cell coordination, thus improving cell coverage. UEs near a center of a cell may also obtain gains, thus improving cell capacity. In a cellular deployment, the area over which a UE can be served with acceptable signal quality without triggering a handoff is referred to as the coverage. System capacity is another metric can be used to measure the efficiency of a cellular deployment and is defined as the throughput in bits per second that can be achieved over a carrier. For example, the coverage can be specified by a signal to interference plus noise ratio (SINR), Γ5, where Γ5 is determined such that 5% of connected UEs experience a received SINR of less than or equal to Γ5, whereas system throughput can be specified by a SINR, Γ50, where Γ50 is determined such that 50% of connected UEs experience a received signal to noise ratio of less than Γ50. The identification of UEs that may benefit from inter-cell coordination may be based on measurement reports sent to a base station by a UE. Generation and transmission of a measurement report from a UE may be based on measured signal quality received by the UE from neighboring cells.
Ideally, the number of measurement reports from a UE should be triggered by an event such that bandwidth consumption on the uplink from the UE to a serving cell is minimized. Further, the event that triggers transmission of a measurement report should be such that transmission of the measurement report occurs only if the UE can benefit from an inter-cell coordination technique.
In existing long term evolution (LTE) standards, the user equipment (UE) makes measurements of signal quality of signals received from a plurality of neighboring cells. The measurements are triggered based on network defined events. A list of such events is predefined in the standards, using reference signal received power (RSRP) and reference signal received quality (RSRQ). For example, the following RSRP-based metric can be used to trigger a measurement report:Γ=(Rl−R0)>η  (1)where Ro is the RSRP of the serving cell and Rl is the RSRP of the strongest interfering cell, as these two parameters are measured at the UE. The threshold, η, is set to regulate the number of reports sent by a UE. When the difference between Rl and Ro is below the threshold, η, the UE reports the RSRP measurements of Ro and an ordered list of RSRPs of a predetermined number of neighbor cells to the serving cell. The number of RSRPs of the neighbor cells is configured by the serving cell. The RSRPs of the neighbor cells are ordered in decreasing order of RSRP. The UE may also report RSRQs of the serving and neighbor cells, as specified in the serving cell's measurement report configuration. For inter-cell coordination, the threshold, η, may be set to a negative value to ensure that the decision to perform inter-cell coordination does not interfere with a handover decision. Note that the above metric may be modified using RSRQ instead of RSRP. However, the performance results follow the same trend since RSRQ can be interpreted as a scaled version of the RSRP. Herein, RSRP measurements and RSRQ measurements will be referred to as signal quality measurements. As specified in the 3GPP TS 36.331: Radio Resource Control specification, other parameters such as hysteresis, frequency and neighbor cell specific offsets can also be included in the metric computed at the UE.
As depicted in FIG. 1, the inter-cell coordination region 2 may be triggered whenever the RSRP with respect to the neighbor cell is within η deci-Bels (dB) of the RSRP with respect to the serving cell. The margin, η, determines the number of measurement events reported by the UEs to the serving cell. The inter-cell coordination can be extended until the handover is triggered, i.e., inter-cell coordination can be operational when the RSRP of a neighbor cell is within [R0−ηH, R0+η], where ηH represents the RSRP threshold for handover. FIG. 2 shows results of gain in geometry that can be obtained at the UE by enabling an inter-cell coordination process, e.g., muting the radio resources from the dominant interfering cells. The term “geometry” is a term of art that relates to long term signal to interference plus noise ratio (SINR) when all cells are transmitting at a constant power level. A gain in geometry is related to a gain in SINR.
The cumulative distribution functions (CDF) shown in FIG. 2 expectedly demonstrate that there is significant gain by muting the dominant interfering cells. Curve number 4 is the CDF when there is no ICIC, solid curves numbered 6 are the CDF for one, two and three interferers using limited feedback, i.e., when the threshold, η, is used to limit measurement reports from the UEs. The dashed curves numbered 8 are the CDF for one, two and three interferers suppressed using 100% feedback, i.e., when the measurements reports are sent periodically from all UEs without restriction. Note that the gains obtained by limited measurement feedback (curves numbered 6) are confined to the lower geometry region. In the simulation results of FIG. 2, η is set to 4 dB, which corresponds to 25-30% UE measurement feedback. Therefore, it is evident that the currently defined triggering event of equation 1, may be ideally suited for handoff decisions—such as identifying UEs experiencing inferior channel conditions—but is less desirable for triggering inter-cell coordination. Thus, events defined by current LTE standards may generate unnecessary measurement reports and/or the reported measurements may not be useful to exploit the full benefits of inter-cell coordination.